Many industrial applications of chemicals require high purities and minimal storage of such high purity chemicals. Some chemicals are desired in mixtures of commonly commercially available chemicals. Diluted forms of such chemicals are desired such as the semiconductor industry's need for diluted ammonium hydroxide in 28 to 30 wt. % in deionized water. Such chemical mixtures require blending of ammonia gas and deionized water in a precise manner. Alternatively, mixtures of ammonium hydroxide and hydrogen fluoride or hydrogen chloride are desired.
To facilitate ease and economy of use, operators have begun to generate or blend chemicals at or near the point of use, which can be referred to as on-site generation. When gaseous ammonia and deionized water are blended to make aqueous ammonium hydroxide, the process is sometimes referred to as gas to chemical generation. Attempts to generate chemical blends at or near the point of use have been made in the prior art, but purity problems continue to exist, such as when blending is not precise or when blended chemical picks up impurities during transit to storage or during extended storage prior to use.
U.S. Pat. No. 5,522,660 discloses a process and apparatus for mixing deionized water and a chemical in which the blended chemical is sensed for compositional properties downstream of the mixing and storage tank and additional chemical is added to the extent the sensed values do not meet predetermined values. Chemical is mixed with the deionized water only by recycle to the mixing and storage tank after the blended chemical has been sensed. This is an awkward manner in which to blend chemical, in which the blending initially occurs in a process line downstream of the mixing and storage tank. Continuous recycle is not envisioned and blending and storage is performed in one vessel.
International Patent Appln. No. WO 96/39651 describes a process and apparatus for mixing ultra high purity chemicals for semiconductor usage on-site. Various chemicals can be blended in a blend tank and delivered to a finished product storage tank. Blended chemical from the blend tank can be sensed prior to delivery to the finished product storage tank and the blend adjusted based upon such sensing. A process line connects the blend tank to the chemical line downstream of the sensor, but recycle is not specifically addressed and chemical delivered to the finished product storage tank cannot be recycled for reblending or repurification.
Other prior art of interest includes: U.S. Pat. No. 5,148,945; U.S. Pat. No. 5,242,468; U.S. Pat. No. 5,330,072; U.S. Pat. No. 5,370,269; U.S. Pat. No. 5,496,778; U.S. Pat. No. 5,426,944; U.S. Pat. No. 5,539,998; U.S. Pat. No. 5,644,921; International Appln. No. WO 96/39265; International Appln. No. WO 96/39237; International Appln. No. WO 96/39263; International Appln. No. WO 96/39264; International Appln. No. WO 96/39266; International Appln. No. WO 96/41687; Peters, Laura, "Point of Use Generation: The Ultimate Solution for Chemical Purity", Semiconductor International, Jan. 1994, pp 62-66; and "Products in Action; Gas-to-Chemical Generation System Reduces ProcessChemical Costs", Microcontamination, June 1994, pp. 79-80.
The prior art has proposed various on-site gas to chemical and chemical to chemical generators and mixers to provide high purity chemical at an industrial use site, such as a semiconductor fab; however the prior art has failed to provide adequate methods and apparatus to repeatedly or continuously purify or filter and monitor compositional properties of chemical that is generated, as well as chemical that is in storage awaiting use after generation. The present invention overcomes the drawbacks of the prior art by providing repeated or continuous purification or filtering and monitoring of not only just-generated blended chemicals, but also chemical which is in storage awaiting use, so as to maintain the purity and compositional properties of the produced chemical at all times, including during periods of low utilization or no utilization, which capabilities will be set forth in greater detail below.